This invention relates generally to magnetic resonance imaging (MRI) of tissue having a plurality of tissue species with different relaxation times, and more particularly the invention relates to separation of the species from analysis of MRI relaxation curves using transient steady-state free precession (balanced SSFP) imaging.
Magnetic resonance imaging (MRI) is a widely used medical imaging modality that provides excellent soft-tissue contrast with arbitrary scan-volume orientations. Unlike X-ray computed-tomography or ultrasound, whose contrast is based only on the transmission or reflection properties of tissue, MRI generates contrast from a variety of physical properties of tissues including relaxation, chemical-shift, diffusion and proton density.
Clinically, one of the most useful MRI contrast mechanisms is T2-contrast, which arises from differences in the spin-spin relaxation time (T2). T2-contrast has numerous uses including distinguishing malignant and benign tumors, imaging spinal abnormalities, vascular imaging and for diagnosing meniscal tears in the knee. Quantitative measurement of the T2 spectrum, or T2-relaxometry, is also useful for a variety of clinical applications including cartilage imaging and imaging of multiple sclerosis (MS).
T2-relaxometry is discussed by Harrison et al. in “Magnetization Transfer and T2-Relaxation Components in Tissue,” Magnetic Resonance in Medicine 1995; 33: 490-496. Many tissues are known to exhibit multi-component T2-relaxation that suggests some compartmental segregation of mobile protons on a T2 time scale. Magnetization transfer (MT) is another relaxation mechanism that can be used to produce tissue contrast in MR imaging. The MT process depends strongly on water-macro molecular interactions. To investigate the relationship between multi-component T2-relaxation and the MT process, multi-echo T2 measurements have been combined with MT measurements. For example, in muscle, short-T2 components show greater MT than long-T2 components while for white matter, MT measurements were identical for two major T2 components, apparently because of exchange between the T2 compartments on a time scale characteristic of the MT experiment.
Whittal and Mackay, “Quantitative Interpretation of NMR Relaxation Data,” Journal of Magnetic Resonance 84, 134-152 (1989), discloses a least-squares and linear programming algorithms for the interpretation of NMR relaxation data in terms of a spectrum of relaxation times.
In the case of MS, the exact T2-spectrum is not as important as the fraction of the spectrum that appears in each peak. Measurement of the relative peak sizes is a good indicator of the myelination of white matter in the brain, and an indicator of MS. Currently, this measurement is performed using a multi-echo Carr-Purcell Meiboom-Gill (CPMG) sequence, see Whittal and Mackay, supra. In the CPMG sequence, the minimum echo spacing is on the order of 10-15 ms to allow for crusher gradients and 180° refocusing RF pulses.
The present invention presents a new method for quantitatively separating tissue based on relaxation time differences with shorter repetition time and improved temporal resolution.